Objectives: Pulmonary parenchymal changes appear many years after the start of exposure to asbestos and their progression has been observed to continue many years after cessation of the exposure. The aim of the present analysis is to assess the risk of progression of radiological changes in the respiratory system, based on long-term follow-up of groups of workers occupationally exposed to asbestos dust. Materials and Methods: The analysis included 3144 individuals voluntarily applying for a medical examination in 2000-2010, who had performed at least two radiographs of the chest. Probability of progression in subsequent studies was based on the evaluation of radiographs for the presence of small, irregular type s, t, u opacities in the parenchymal lung tissue with profusion categories as specifi ed in the classifi cation of ILO, 1980. Multistate hidden Markov model was used in the statistical analysis. Results: In the group of patients, small irregular opacities with a profusion of 1/1 or higher occurred in 640 (20%) patients in the fi rst examination, and in 918 (29%) patients in the last examination. No parenchymal changes were observed in 1360 (43%) patients in the fi rst examination (opacity profusion category 0/0), and in 645 (20%) patients in the last examination. The risk of progression of radiographic changes in former workers of asbestos processing plants is higher in smokers, from profusion category 0 to 1 - by 30% (HR = 1.30, 95% CI: 1.15-1.47), from profusion category 1 to 2 - by 50% (HR = 1.50, 95% CI: 0.92-2.45). The risk of progression of parenchymal changes decreases in the workers employed in subsequent calendar years: for category changes
from 1 to 2, the decrease is nearly 40% for every 10 years of subsequent employment (HR = 0.60, 95% CI: 0.45-0.80). The probability of the progression increases with longer latency period: for change of category from 1 to 2, the increase is about 20% (HR = 1.19, 95% CI: 1.00-1.42) per 10 years of employment for blue collar workers. The risk of the 1 to 2 category progression is signifi cant, nearly 2.5-fold higher in the workers employed in the asbestos-processing textile industry. Conclusions: Our results show that the risk of progression of pulmonary parenchymal fi brotic processes was higher in the
smokers, workers employed during the earlier calendar years of the discussed period, those with longer latency period, blue collar workers involved directly in the production processes, and employees of the asbestos cement and/or textile industries.